1. Field of the Invention
This invention relates to a method of heat-treating a fluoride crystal, a method of producing an optical part, and an optical apparatus, and more particularly to a method of heat-treating (annealing) a fluoride crystal such as calcium fluoride, barium fluoride and strontium fluoride.
2. Related Background Art
The use of ultraviolet light has been increased in the field of laser processing, for example, in photolithography which is used in the production of semiconductor devices such as a microprocessor, a memory, a system LSI, an image sensor, a light-emitting element and a display element.
With such an increase, a material other than quartz glass has been demanded for use in optical parts such as lens, a prism, a half mirror and a window material.
Fluoride crystals such as calcium fluoride, barium fluoride, strontium fluoride and lithium fluoride are preferably used for optical parts as a highly light-transmissive material.
In the production process of a fluoride crystal, annealing is conducted by heating the fluoride crystal almost to a temperature of the melting point of the fluoride crystal without melting the fluoride crystal so that a stress generated in the inside of the crystal can be relaxed.
The double refractions of the fluoride crystal before and after the annealing are measured and it is found that double refraction is significantly decreased in the fluoride crystals subjected to the annealing.
However, in order to enhance the annealing effect, for example, in the case of calcium fluoride, the fluoride crystal is subjected to the annealing of increasing a heating temperature up to 1000xc2x0 C. or more, whereby turbidity and coloration sometimes occurred.
After the investigation of the cause, it was found that the occurrence of such turbidity and coloration was due to oxygen adsorbed by and metal impurities attached to the surface of the fluoride crystal. Fluoride crystals taken out from a crystal-growing furnace after crystal growth are carried in the atmosphere so as to be placed in a heat-treating furnace, and oxygen and impurities may attach to the crystals during this carrying step.
When such oxygen and impurities are heated to a temperature of 1000xc2x0 C. or higher, they diffuse in and react with the calcium fluoride. This may cause turbidity and coloration. Such a phenomenon also occurs in the other fluoride crystals whose melting points are different from that of calcium fluoride.
Accordingly, an object of the present invention is to provide a method of heat-treating a fluoride crystal and a method of producing an optical part without easily generating turbidity and coloration in the crystal and the part.
Another object of the present invention is to provide an optical apparatus excellent in optical properties, which is produced by using the optical part obtained by the above heat-treating and production methods.
A further object of the present invention is to provide a method of heat-treating a fluoride crystal which comprises the steps of making an atmosphere of an inert gas and/or fluorine-based gas having a pressure not lower than atmospheric pressure in a heat-treating furnace in which a fluoride crystal is placed, and heating the fluoride crystal in the above atmosphere.